1. Field of the Invention
This invention relates to an improved tonometric measuring device for detecting excessive intraocular pressure such as that associated with glaucoma.
2. Prior Art
The current diagnosis of glaucoma relies on the detection of increased intraocular pressure. Numerous methods for making such measurements have been utilized, including both invasive and non-invasive techniques. The primary non-invasive instruments have generally involved a tonometric measurement over the cornea, resulting in discomfort to the patient and incapacitation during the period of measurement. Where the measurement is made by appanation of a small surface area, the contacting diaphragm is usually of a compliant form, as opposed to being essentially noncompliant. The use of the compliant form in connection with the pressure applied results in a gradual decrease in IOP due to forced fluid escape during the measurement process. Such fluid escape attenuates accuracy in continuous monitoring methods due to the cumulative loss of fluid pressure. Furthermore, the variation of intraocular pressures occurring over an extended period of time or with change in activity makes isolated measurements less informative as to the true condition of the patient. For example, a patient may have a moderately high pressure reading while in a prone or restful position, but experience more dangerous pressure levels when involved in exhilarating or emotional activities.
Most current instrumentation is not adaptable to continuous measurement because a technical assistant is required to be present or the vision of the patient is impaired, thereby restricting activity or movement. A detailed discussion of the prior art relating to noninvasive measurement of IOP is found in Macri and Brubaker, "Methodology of Eye Pressure Measurement," Biorheology 6.37, 1969.
The continuous measurement of intraocular pressure over longer periods of time is more closely comprehended by a contact lens measuring device which is worn over the cornea in a manner similar to normal contact lenses. This device is described in Green and Gilman, "Intraocular Pressure Measurement with Instrumented Contact Lenses," Investigative Ophthalmology, 13:299, April, 1974. Essentially, it comprises a flush-fitting silastic gel contact lens implanted with strain gauges for measuring changes in the meridional angle of the corneoscleral junction. This application of tonometric measurement relies on the theoretically predicted angular change of 0.020 to 0.016 radians per mm Hg change of intraocular pressure.
The use of the contact lens material permits the patient to enjoy unobstructed vision during measurement; however, the sensitivity of the cornea results in limitations to continuous use caused by eye irritation and inflamation similar to that experienced with the prolonged wearing of regular contact lenses. Furthermore, the measurement of intraocular pressure with reference to the corneoscleral junction requires an in situ calibration which may cause some discomfort and inconvenience to the patient.
What is needed, therefore, is a device for continuously measuring intraocular pressure over an extended period of time without causing irritation to the eye or requiring special in situ calibration techniques, while retaining the requisite noninterfering characteristics which permit the patient to engage in his normal activity.